This invention relates to flat alkaline cells in general, and more particularly to a flat alkaline cell construction wherein the active elements are enclosed within a sealed, liquid-impervious plastic film envelope and wherein the envelope is provided with both positive and negative terminal connections and a third terminal connection for an auxiliary electrode.
In our copending application Ser. No. 456,837 entitled "Flat Alkaline Cell Construction," filed Apr. 1, 1974, of which this application is a Continuation-In-Part, there is disclosed and claimed a flat alkaline cell wherein at least a pair of flat electrode elements of opposite polarity having a porous separator containing an alkaline electrolyte interposed therebetween are arranged in the form of a conventional electrode stack assembly. A current collector which may be a flat metal plate or foil is disposed adjacent to and in electrical connection with one of the pair of electrode elements at one end of the electrode assembly. The electrode assembly is enclosed within a sealed, liquid-impervious plastic film envelope having an opening in one wall thereof exposing at least a portion of the current collector. A thin layer of an adhesive sealant is interposed at the interface between the current collector and the inner surface of the wall of the sealed envelope surrounding the opening. This layer of adhesive sealant tightly adheres and seals together the collector and the envelope wall and prohibits leakage of the alkaline electrolyte through the opening from inside the cell. Preferably, the layer of adhesive sealant covers substantially the entire face of the current collector, except for a small area coinciding with the opening in the wall. Suitable means may be provided for making external electrical connection with the exposed portion of the current collector such as by attaching a wire lead thereto.
Alkaline electrolytes are notoriously known for their ability to readily wet most metal and plastic surfaces and to creep seals conventionally used in current-producing electrochemical cells. Our flat cell construction as described above effectively solves this problem through the use of an adhesive sealant which is non-wettable by the alkaline electrolyte. The thin layer of adhesive sealant used to tightly bond the current collector to the wall of the plastic film envelope actually resists creepage of the electrolyte past the sealing interfaces and out through the opening in the wall. Preferably, the adhesive sealant is chosen from the class of compounds known as "fatty polyamides," although other adhesive sealants which are not readily wet by the alkaline electrolyte can also be used.
Still another advantage of our flat cell construction resides in the provision of an extended leakage path over which the electrolyte must travel in order to escape from the cell. This extended leakage path comprises the sealing interface between the current collector and the inner surface of the wall of the sealed envelope, i.e., the leakage path traverses the shortest distance from the opening over the width or length of the collector.
In a typical flat cell construction made in accordance with our above-referred to copending application, the sealed envelope is advantageously formed by a heat shrinkable plastic film tube which is heat shrunk down around the side walls of the electrode assembly and which overlaps the marginal borders of a pair of flat current collectors, one of each of which is disposed adjacent to an end of the electrode assembly. The pair of current collectors and the portion of the heat shrunk tube overlapping the marginal borders are tightly adhered together by a thin layer of the non-wetting adhesive sealant. The centermost portion of one of the pair of current collectors which is left exposed at one end of the electrode assembly constitutes the positive terminal connection while the centermost portion of the other of the pair of collectors which is left exposed at the opposite end of the electrode assembly constitutes the negative terminal connection of the cell. The arrangement of the terminal connections is such that a plurality of the individual flat cells may be stacked together with the positive terminal connection of one cell making electrical connection with the negative terminal connection of an adjacent cell via a wire lead or the like attached thereto to constitute a series-connected battery. Flat cells with this arrangement of the terminal connections can also be assembled into parallel and series-parallel connected batteries.
It has already been proposed in the prior art to employ so-called "auxiliary electrodes" in current-producing electrochemical cells. These auxiliary electrodes are used in conjunction with the working electrodes, i.e., positive and negative electrodes, to perform certain valuable functions in operation of the cells. Such auxliary electrodes have been used, for instance, in rechargeable cells to detect or sense the presence of excessive amounts of either oxygen or hydrogen gas that may be generated under certain conditions such as when the cells are overcharged.
In U.S. Pat. No. 3,462,303 to H. Reber, there is disclosed a sealed rechargeable cell wherein an auxiliary electrode is maintained in contact with a gas space and a liquid electrolyte. The auxiliary electrode will form with the negative electrode of the cell a voltage differential the value of which will be dependent on the partial oxygen pressure in the gas space of the sealed cell. When the cell is subjected to overcharging, the partial oxygen pressure in the gas space will rise, a change in the voltage differential will occur and this change is utilized for actuating control devices for terminating the charging current and thereby prohibiting the build-up of an excessive gas pressure inside the cell.
Basically the same auxiliary electrode arrangement may be used in a rechargeable cell such as described above to sense the presence of hydrogen gas in the cell. In this instance, the auxiliary electrode which will form with the positive electrode of the cell a voltage differential whose value will be dependent on the partial hydrogen pressure in the gas space under conditions where hydrogen gas may be evolved during operation of the cell.
Auxiliary electrodes may also be incorporated in current-producing electrochemical cells as a voltage reference device. It is possible for instance to electrochemically couple the positive or negative electrode of a rechargeable nickel-cadmium cell to an auxiliary reference electrode and study the discharge behavior of either electrode independently of the other. Auxiliary electrodes in these applications are a valuable tool to the researcher since he can carry out his studies without having to dismantle the cell construction.
Provision must of course be made in the sealed assembly of the current-producing electrochemical cell for making external electrical connection with the auxiliary electrode. Such means usually comprises a separate or third terminal connection in the sealed assembly in addition to both the positive and negative terminal connections. Basically the same type of construction is used for the third terminal connection regardless of whether the auxiliary electrode is employed as an oxygen or hydrogen sensing electrode or voltage reference electrode.
The principle object of this invention is the provision of a flat alkaline cell construction of the character described in our above-referred to copending application wherein there is provided both positive and negative terminal connections in at least one wall of the plastic film envelope together with a third terminal connection for an auxiliary electrode.